THE METROPOLITAN MUSEUM OF ART 


THE 
RESTORATION 
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ANCIENT BRONZES 
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OTHER ALLOYS 














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THE METROPOLITAN MUSEUM OF ART 


The Restoration 
of 
Ancient Bronzes and Other Alloys 


Biv 


Corin G? Fink, PH: D. 


Professor of Electro-Chemistry at Columbia University 


AND 


(HARTES fi, ELDRIDGE, 6.9. 


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First Report 


NEW YORK : MCMXXV 


A a a 1 
COPYRIGHT BY 


THE METROPOLITAN MUSEUM OF ART 


MAY, 1925 








INTRODUCTION 


Ir 1s a general impression among visitors to museums that once 
a work of art has been properly installed, once it has been set on 
a pedestal or hung upon a wall, and more especially if it has been 
protected by glass, its life is assured for an indefinite future. So 
far is this from being the fact, however, that exactly the con- 
trary might almost be said to be true. Many objects bring with 
them the germs of deterioration if not of actual destruction, 
others though sound when they are received are highly sensitive 
to relative degrees of heat and cold, moisture and dryness, and 
to the atmospheric impurities of our modern cities, never so 
serious a menace as they are today. These dangers are not con- 
fined to any one material or class of materials, indeed there are 
very few—if any—which can be regarded as immune from one 
or another of them; and when we add to this the knowledge that 
one “diseased” specimen in a show-case may and frequently does 
infect others, it will be realized that eternal vigilance is the price 
at which a curator keeps his collections in condition, though he 
is often at his wits’ end to know how to arrest or prevent the 
disaster with which he finds himself threatened. 

Happily a few chemists have been coming to our assistance 
during recent years, by making a serious study of our problems 
in the light of the advances which have been made during this 
period in their own science, especially electro-chemistry, with 
the promise of results which we should formerly have believed 
impossible. This has been notably the case in the laboratory 
established in the British Museum by the Government Depart- 
ment of Scientific and Industrial Research under the able direc- 
tion of Dr. Alexander Scott, F.R.S. Following that example 


the Trustees of the Metropolitan Museum of Art determined a 
little more than two years ago to broaden this field of endeavor 
by undertaking independent investigations in one type of ma- 
terial which presents particularly vexatious problems; namely, 
ancient bronzes. The evils with which these are most generally 
afflicted and which are the most difficult to overcome may be 
roughly divided into two classes. One is the corrosive green 
crust with which objects in bronze or other copper alloys are apt 
to become thickly coated through long burial in the earth, and 
which not only completely hides the details of the modeling 
and the engraved designs, but frequently eats deeply into the 
surface of the metal itself, thereby rendering the object value- 
less, and sometimes destroying it altogether. ‘The other is what 
is now familiarly known among collectors as the “bronze dis- 
ease.’ Both of these highly disfiguring and destructive agents 
are described and discussed in the following pages, as well as the 
difficulties in dealing with them. 

In entering upon our task we were fortunate in having the 
interest and assistance of the chemical authorities of the Na- 
tional Research Council, under whose guidance we secured the 
services of Professor Colin G. Fink of Columbia University, 
whose experience in the study of metals made him singularly 
fitted to undertake investigations of the kind required. The 
Museum is also under great obligation to Columbia University 
for its friendly codperation in allowing us the free use of its 
chemical apparatus, and in setting aside a special laboratory 
exclusively for our investigations. Without these combined 
privileges it would have been difficult indeed for us to pursue 
them. 

With the assistance of Mr. Charles H. Eldridge, Dr. Fink 
began his studies and experiments in January, 1923, and al- 
though a final solution of all the problems involved has not yet 
been reached, what has been achieved thus far is so extraordi- 
nary as to warrant the publication of a preliminary report giv- 


ing the results accomplished up to date, inasmuch as the Trus- 
tees of the Museum desire to share the benefits they are deriving 
from these researches with the whole scientific world, a desire in 
which Dr. Fink joins in the most liberal spirit. In doing this we 
are again following the example of the British Museum. 

As to the results, even a layman can see from the illustrations 
in this report, showing objects before and after treatment, that 
so far as the removal of the crust is concerned these may be 
fairly described as sensational. We are now well on the way to 
salvaging brilliantly thousands of bronzes which might other- 
wise have been ruined by crude attempts at ‘“‘cleaning”’ or dis- 
carded altogether as worthless. The artificial patination of such 
objects as may require it after treatment, and the cure or pre- 
vention of the bronze disease, still call for further study, but 
considerable progress has been made in both, and we are justi- 
fied in believing that satisfactory solutions are not far distant. 

In conclusion, I should like to offer a word of caution. The 
museum official or collector who hopes to find in this report a 
ready-made formula for treating bronzes which anybody can 
apply will be disappointed. While Dr. Fink and Mr. Eldridge 
have shown that ancient bronzes can be cleaned and preserved 
they have demonstrated with equal force that this can be done 
only by a highly trained expert who like a physician watches 
and deals instantly with every symptom that develops during 
his treatment, as otherwise his patient may be lost instead of 
cured. The conclusion is obvious. The only way of putting the 
knowledge gained by these investigations to practical and gen- 
eral use is by the establishment of a central bureau or labora- 
tory, available to the country at large, to which museums, pri- 
vate collectors, and dealers may send their specimens for treat- 
ment with the assurance that this will be of the highest scientific 
character. With the financial support of one of our great Foun- 
dations the National Research Council might be well equipped 
to effect the organization of such an establishment, and if its 


investigations were to be extended to other materials than 
bronze it would be of incalculable value in preserving our works 
of art for posterity. Is it too much to hope that such a consum- 


mation may be brought about ? 
EDWARD RosBINson, 


Director. 


March 2, 1925. 


RESTORATION OF ANCIENT BRONZES 
AND OTHER ALLOYS 


REPORT OF AN EXPERIMENTAL RESEARCH 
CARRIED OUT AT COLUMBIA UNIVERSITY 
FOR THE METROPOLITAN MUSEUM OF ART 


HE restoration of objects of antiquity has been practised 

for a good many years but the methods applied have been 
almost invariably of a haphazard “‘hit or miss” type. Further- 
more, it has usually been the practice in the past to keep secret 
the methods applied in restoring or trying to restore a valuable 
find. Accordingly, it is not surprising that there are very few 
references in the literature that give any details whatsoever on 
“Methods of Cleaning or Restoring.” It has been unfortunate 
that men who undertook to clean, repair, and restore usually had 
no artistic or scientific knowledge or appreciation but were pri- 
marily interested in the sum to be realized upon completion of 
the “cleaning.” That methods employed by the various “anti- 
quarians”’ are not at all alike and are conducive of very diver- 
gent results is everywhere evident: we need merely compare a 
collection of bronzes found in the very same locality but treated 
by different agents and observe the widely different results. 
There has almost always been a desire on the part of the 
“restorer” to bring out details in design and yet not make the 
object appear as though it were of recent origin. In looking over 
collections of ancient bronzes we furthermore observe that 
methods of cleaning applied were in many cases of a most rad- 
ical kind. For example, a number of ancient bronzes have been 
studied by us where there is little doubt that strong chemical 
reagents, such as mineral acids, were used to remove the crust. 
In other specimens, chisel marks were observed, indicating 


§ RESTORATION OF ANCIENT 


plainly that the method of removing the crust or hard outer 
layers—so common with bronzes that have lain buried in the 
soil for ages—was of a very crude mechanical sort. Ruined or 
partially ruined bronzes such as these encouraged the present 
writers to search for a more reliable method of cleaning and 
restoring. 

At the time the present research was undertaken there were 
two rather distinct studies presented for investigation: 

1. An improved and more general method of restoration. 

2. The best means of combating the bronze disease. 

The lack of scientific publications on the restoration of an- 
cient bronzes is probably to be attributed to the difficulty in 
developing a truly scientific process, due primarily to the very 
wide diversity in the composition of the bronzes and other an- 
cient alloys. A method carefully worked out and applicable to 
a certain specimen of bronze is not necessarily applicable to any 
of the other bronze objects in a particular group or acquisition. 
Even when the composition of the bronze proper is identical— 
or very nearly so—in two specimens, it does not follow that the 
method which gave satisfactory results in the first specimen will 
give equally satisfactory results in the second. As a concrete 
example, the crust of the first object may be very thin and 
readily removed with a weak chemical reagent whereas in the 
second, the crust may be hard and much thicker and the weak 
reagent have little or no effect on the crust. Furthermore, dif- 
ferent sections of the same bronze object are often differently 
corroded, and one of the older methods, such as a dip in nitric 
acid, would not affect the two sections alike: it might “clean” 
one and badly pit the other. 

Another inevitable obstacle in beginning the pursuit of a 
purely scientific research on the restoration of bronzes and other 
metal objects of antiquity, 1s the absolute lack of information 
as to whether, upon the removal of the outer crust or mineral- 
ized layers of the bronze, we shall find any artistic design at all. 


BRONZES AND OTHER ALLOys 9 


Accordingly, a really serviceable and generally applicable 
method of restoration must not be dependent upon differences 
in composition of the alloy or of the incrustation or patina, but 
must work well with all; and the resultant product must not 
leave any doubt in the mind of the restorer that perhaps some 
other method might have been more satisfactory. 


The Process of Corrosion 


Before briefly outlining the method we finally developed and 
adopted, and found very satisfactory indeed for a large variety 
of bronze compositions in various stages of decomposition, it is 
well to consider what takes place in the process of corrosion 
which gives rise to the green, blue, brown, and red layers of the 
outer shell or crust covering the bronze. Upon examination we 
find that these outer layers consist primarily of various copper 
minerals such as the carbonate (green malachite or blue azur- 
ite) the chloride and oxide or oxychloride and occasionally the 
sulfate and sulfide. Then there are tin and lead minerals present 
in some incrustations. Bronzes containing silver and other silver 
alloys often contain silver oxide or silver sulfide, in the crust; 
but in the presence of the less noble metals, such as copper, tin, 
zinc, and lead, silver in the crust is usually present as metal. 
Gold is almost invariably unattacked. The copper as carbonate 
or other compound in the mineral crusts is derived almost al- 
ways from the underlying bronze. There are specimens that 
indicate that the metal in the crust has been derived from out- 
side sources, but these cases are comparatively rare. 

The nature and causes of the corrosion of ancient bronzes 
have been investigated in the past by Rathgen,* Frisch,” Mar- 
gival,’ and others. The chief corroding agents are usually chlo- 


I. The Preservation of Antiquities, by Dr. Friedrich Rathgen, Director of the Berlin 
Museum, 1905, English translation published by the University Press, Cambridge. 

2. Natural Patina and Artificial Patina, by Ch. Frisch, Revue de Chimie Industrielle, 
Vol. 15 (1904), pages 169-174. 

3. Bronze Patinas, Francois Margival, Revue de Chimie Industrielle, Vol. 22 (1Q1T), 
pages 304-310, 331-332. 


10 RESTORATION OF ANCIENT 


rides and nitrates present in moist soil. The constituents of the 
bronze are slowly converted into an outer hard green crust con- 
sisting of oxychlorides and oxycarbonates of copper mixed with 
a certain quantity of tin compounds, chiefly tin oxide; and 
underlying this green crust is a layer of copper oxide, and very 
often below this again, a core of metal. In extreme cases the 
corrosion has gone all the way through the specimen, and noth- 
ing of the original bronze metal remains. In such a case we have 
no longer a bronze object, but a body, consisting of a shell or 
crust of copper compounds, usually green in color, covering a 
soft brittle core of copper oxide, red or brown in color. 

Apart from the corrosive action of the chlorides and other 
salts in the soil, the prime important soil constituent necessary 
to bring about the most frequently met with type of corrosion of 
ancient bronzes is water in which the chlorides or other salts are 
dissolved. Water present merely as moisture in the soil in which 
the bronze lies buried is sufficient to bring about mineralization 
of the surface of the bronze and eventually of its entire body. 
We have had under investigation a number of bronzes that had 
been completely mineralized in comparatively dry locations. 

It is generally conceded today that the corrosion of metals 
and alloys is due to an electrolytic process, and it occurred to us 
that if this electrolytic process of corrosion could be reversed a 
method might be devised whereby the metal compounds in the 
crust would be reduced back to metal. This line of attack of the 
problem seemed most hopeful, since other methods, such as dip- 
ping the bronze into strong mineral acids, are far too radical and 
seldom produce satisfactory results. 

It will be appreciated at once that any acid method is very 
difficult to apply, since it is almost impossible to regulate the 
action of the acid in such a way as to avoid the pitting of the 
underlying metal. Furthermore, the metal contained 1n the com- 
pounds of the incrustation is removed by the acid whereas in the 
ideal case it should be reduced and put back into place. 


BRONZES AND OTHER ALLOYS 11 


L ype of Bronzes Investigated 


The word “bronze” conveys to the minds of most people the 
idea of an everlasting and never-corroding metal, but such a 
bronze does not exist. Whenever specimens of very ancient 
bronze are met with that are in an excellent state of preserva- 
tion this is due to favorable conditions of location, such as 
an absolutely dry grave or tomb. Many bronze articles that 
throughout the ages have lain embedded in the ground in con- 
tact with corrosive reagents are badly encrusted and outwardly 
deformed. It is with corroded bronzes of this latter type that we 
are primarily concerned in this report. Some of these bronzes are 
covered with a crust so massive that recognition or identification 
is almost impossible. In other cases, the metal constituents have 
been completely mineralized. With others again the corrosion 
crust is so thick that thin objects, such as mirrors or plates, are 
warped, owing to unequal expansion. Along with such extreme 
crust formation, parts of the original bronze object will often 
be entirely corroded away, the metal having disappeared even 
beyond the outer crust. 


Object of the Investigation 


The object of our investigation was to start with specimens 
of badly corroded bronze as described above and to so treat 
them that the ugly crust should be removed and at the same 
time to restore or to preserve intact any detail of shape, design, 
engraving, tool mark, or other signs of workmanship remaining 
on the specimen as found. Now this may appear an almost 
impossible task; and it would be, except for one redeeming fea- 
ture. Almost always the details of design or workmanship of the 
original article have been found to be preserved and copied 
exactly by the layer of copper oxides which underlies the ugly 
and deforming green crust. Having established this, our first 
step was to find a method to remove or reduce the green crust 


12 RESTORATION OF ANCIENT 


and to expose intact the oxide layer, partially or wholly reduced, 
beneath. We have developed a method which has given very sat- 
isfactory results, and which will be described more fully below. 
Our method of restoration has been successfully carried out 
with specimens in which the oxide layer still has a true metal 
core, and likewise with specimens in which the metal of the in- 
terior is completely corroded, and where only a core of weak 
porous oxide remains. In either case, our method of treatment is 
practically the same, except that the completely mineralized 
articles call for extreme care against mechanical breakage. 


The Methods of Other Investigators 


The idea of making ancient bronzes more presentable by re- 
moving the crust is not a new one. It is not uncommon among 
collectors and dealers to treat such a specimen with an acid so 
as to dissolve away the crust. The usual result of such an “acid 
test”? is to dissolve the corroded parts along with some of the 
metal core, leaving a rough, unlovely metallic surface. Fine 
details will be obliterated, and we shall have a “stripped” 
surface, often pitted. 

Other investigators or so-called “dealers” have in the past 
applied purely mechanical methods for the removal of the 
crust. Such methods are, however, most difficult to carry out 
and seldom produce satisfactory results. Again others have used 
the water bath, that is, placing the bronze object into a deep 
dish of water so as to completely immerse the object. By this 
method a soft crust will loosen and gradually drop off after 
days of submersion; if the underlying surface is hard or me- 
tallic and resistant to the action of water very pleasing results 
will be obtained, but if the underlying surface is a porous brown 
or black oxide there is always the danger of destroying the 
details. At all events we have not found this water-bath method 
to be a safe and reliable one and would recommend it only in 
special cases, cases where the bronze or silver object is in a very 


BRONZES AND OTHER ALLOoys ie} 


good state of preservation and merely requires simple removal 
of a thin layer of soil or earthy matter loosely adherent. 

Finkener was one of the first to use an electrolytic method. 
This method is described in detail in Rathgen’s book. Finkener 
used a 2 per cent. solution of potassium cyanide. Under the 
title, Natural Patina and Artificial Patina, Ch. Frisch (loc. cit. ), 
outlines electrolytic methods for the removal of the heavy green 
patina. Later, in 1911, Francois Margival also discusses elec- 
trolytic methods of attack. 

Francesco Rocchi, an Italian, presented a paper entitled 
Contribution of the Experimental Sciences in Art and History, 
in the Rassegna d’Arte, Vol. 7, 1920, pages 258-264. He illus- 
trates the results of an electrolytic method for cleaning and re- 
storing bronzes. Several photographs show objects before and 
after treatment with gratifying results. However, no details of 
procedure are given, nor even the composition of the electro- 


lytic bath. 
British Museum I nvestigations 


In 1919, the Department of Scientific and Industrial Re- 
search of the British Government organized a laboratory at the 
British Museum, and there under the direction of Dr. Alex- 
ander Scott, F.R.S., attention is given to the work of cleaning, 
restoring, and preserving museum objects. The work covers 
prints, enamels, silver, lead, iron, copper, and copper alloys. A 
very interesting booklet was published in 1921, entitled The 
Cleaning and Restoration of Museum Exhibits, bulletin No. 5, 
Department of Scientific and Industrial Research, which cov- 
ered the work up to that date. In 1923, a second report with the 
same title was published, describing Dr. Scott’s most recent 
work. 

In our work at Columbia we have freely used Dr. Scott’s re- 
ports and have received much valuable aid from them. Dr. 
Scott, however, uses ordinary chemical methods for removing 
crust formation and makes no reference to electrolytic methods. 


14 RESTORATION OF ANCIENT 


Results of Investigations at Columbia 


As indicated above, the original purpose of our investigations 
at Columbia University was to remove the unsightly crust by 
any means possible, so as to restore the object to a state more 
nearly as it appeared ages ago. We felt, however, that if we 
could eliminate or reduce this crust and at the same time un- 
cover and preserve the detail underneath, that many museum 
objects now nothing but an ugly green mass could be made pre- 
sentable and attractive. This we believe we have succeeded in 
doing. 

We have given a large variety of methods a most careful trial 
and observed the results. We very soon decided in favor of an 
electrolytic method of attack. The fundamental idea was to 
replace cathodically the metal that had passed into the crust 
rather than to completely eliminate the crust with its metal 
content by corrosive acids or by mechanical means. Our electro- 
lytic method as finally developed 1s of very general application 
and can be used with absolute safety in practically all cases of 
restoration. [his is the one strong argument in favor of the 
electrolytic method of restoration—it is safe. 

In a large number and variety of cases, ancient bronze speci- 
mens were received at our laboratory that were very badly cor- 
roded and extremely fragile. Usually investigation showed that 
the entire metal mass had been completely mineralized. Upon 
submitting the specimen to our electrolytic treatment, we suc- 
ceeded in recovering most of the detail, and in revealing inscrip- 
tions, markings, etc., put on by the early artist. In cases such as 
these mechanical removal of the crust 1s out of the question on 
account of the extreme fragility of the specimen. And an acid 
bath would have destroyed the details. 

In developing our electrolytic process we tried out various 
electrolytes used by earlier workers, as well as other electrolytes 
suggested by our own laboratory observations. We finally de- 


BRONZES AND OTHER ALLOoys By 


cided upon a 2 per cent. solution of caustic soda (Na OH), as 
the safest electrolyte to use. Finkener’s electrolyte of 2 per cent. 
potassium cyanide (KCN), was not found to be satisfactory. 
We do not recommend its use. Potassium cyanide is extremely 
poisonous and its use even by skilled workers is dangerous. Fur- 
thermore, the cyanide solution has a very marked solvent action 
on bronzes, in particular those covered with a delicate copper 
oxide layer, even when the bronze object is made cathode. 


The Method in Detail 


Without any preliminary cleaning, the bronze object to be 
treated is hung as cathode into the 2 per cent. caustic soda solu- 
tion and a low amperage direct current is applied. The object is 
suspended with soft copper wires and is completely immersed 
into the solution. In case the object is very soft and fragile or is 
completely mineralized, fine annealed copper wire is wrapped 
around the object, one to two turns per inch, and electrical con- 
nections are made with several turns of this wire. Where there 
is danger that the object might not hold together upon the re- 
moval of the hard supporting shell, we have found it advisable 
to pack the whole object in clean white sand, after making 
proper electrical connections, and then filling the containers 
with the caustic soda solution. 

The anodes are hung on either side of the object. Iron, duriron, 
and platinum anodes have been used with success. A rectangular 
glass battery jar of one litre capacity serves well as a container 
tor the treatment of small bronzes. For large objects we have 
used stoneware tanks and there is no objection to the use of 
large tanks made of heavy sheet iron welded at the joints. 

For a small object of about two to ten square inches of ex- 
posed surface, the cell is connected in series with a rheostat and 
the 110 volts direct current circuit, so as to send from 0.1 to 0.5 
amperes through the circuit. A slight gassing at the anode will 
occur. Sometimes the crust resistance at the point of contact of 


16 RESTORATION OF ANCIENT 


the copper wire is so high that an appreciable current will not 
pass through the cell at first. Rather than file a clean contact, 
which is hardly ever to be recommended, it is best to start the 
cell at night, allow the solution to slowly penetrate the crust 
and usually by the next morning, the 110 volts potential will 
be sufficient to have broken through the submerged crust with- 
out injury to the surface. Often wetting the copper wire contact 
with electrolyte will break down the dry resistance of the crust 
at that point. The object being treated is always made cathode. 

The action of the electrolysis is to evolve hydrogen at the 
cathode and to so reduce the crust to finely divided or spongy 
copper. This is effectively accomplished in the caustic soda 
solution. At times reguline copper metal will be deposited in 
place. Very low current densities are preferred. 

The reduction of a thin crust 4%. to % inch thick usually re- 
quires three or four days. In the case of clayey crusts it is a 
good plan to change the electrolyte once every twenty-four 
hours. The use of too strong an electrolyte, or too high a current 
density will cause excessive gassing at the cathode (the object 
being treated) and may give rise to warping and falling apart 
of the object unless it is underlaid by a strong metal core. 

Complete reduction is indicated by a free evolution of gas at 
the cathode with comparatively low current values. The object 
is then removed from the solution and carefully washed by 
soaking or steeping in several changes of warm water. This will 
remove all but traces of caustic soda. 

We now arrive at a stage in the process where we must use 
judgment enough to modify subsequent treatment to fit the 
general appearance and physical strength of the specimen. If 
the object treated originally had a hard metal surface under a 
thin sandy layer of crust, we may remove the reduced copper 
film or layer by a gentle brushing with a stiff bristle brush or 
soft brass-wire brush. 

If, however, examination preliminary to electrolytic treat- 


BRONZES AND OTHER ALLoys 17 


ment indicates that no true metallic core is present, we do not 
dare subject the treated surfaces to pressure or friction of any 
kind. In this case a weak acid dip may be necessary and is used 
as follows: 

After thorough washing to remove all but traces of caustic 
soda, carefully dip the reduced object into dilute nitric acid 
(one part acid to four parts of water). There will result a rapid 
action and evolution of gases, as the reduced outer layer of cop- 
per dissolves. By alternately dipping into warm water and into 
dilute acid, we can control the extent of the treatment. As the 
reduced copper dissolves away, we bring into view the hard, 
brownish-colored copper oxide surface which preserves the de- 
tail of design. This oxide layer does not dissolve readily in the 
weak acid used, but takes on a greyish tint (due probably to 
intermixed tin compounds ). Inspection with a magnifying glass 
will show when the treatment has proceeded far enough. It 1s 
an exciting moment when we see the copper dissolve, showing a 
smooth surface, or design, beneath. The oxide surfaces now 
exposed are almost as smooth to the touch as metallic surfaces 
and they will take on a greenish tint or patina when dipped into 
dilute ammonium acetate and dried in an oven at 40° to 60° 
Centigrade (104° to 140° Fahrenheit). 

Sometimes, especially if the original surface shows cracks in 
the patina or conspicuously high lumps or “‘boils” of crust, we 
would find the underlying oxide layer pitted and roughened in 
spots 1f we had used the weak acid dip. Such regions have been 
highly corroded by localized action and the treatment in our 
electrolytic bath causes brown copper to fill the pits. Therefore 
in these cases the weak acid dip is omitted. 


Treatment of Individual Objects 


The best way to show the possibilities of the electrolytic 
treatment is to describe results obtained on various individual 
objects. As the objects were received for treatment each was 


18 RESTORATION OF ANCIENT 


assigned a serial number. Such serial or laboratory number will 
be used to identify the objects in this report. 

The first step in each case was to submit the object to a very 
careful examination. It was measured and weighed; the crust 
or patina was studied and analyzed if necessary, in order to get 
an estimate of the probable composition of the underlying base. 
Detailed records were kept of each object investigated. 


BRONZE VASE (Serial No. 25) 


This is a pear-shaped vase about 3% inches high. Fig. 1a 
shows the vase as received for treatment. As received the object 
was covered with a hard green deforming crust. Here and there 
over the surface were to be found warts or knobs of hard crust. 
Preliminary examination indicated that no metallic core re- 
mained, and that the vase was corroded or mineralized all the 
way through. 

This case was selected to show permanently on one specimen 
the difference in the appearance of the surface before and after 
treatment, and therefore only the lower half was treated. This 
was subjected to cathodic reduction in dilute caustic soda for 
one week. After such treatment the reduced surface was treated 
with weak nitric acid. After removal of the red spongy copper, 
the oxide core was revealed beneath; it was smooth, detailed, of 
a brick-red color, and with the aspect of unglazed earthenware. 
This oxide core was very weak mechanically and required the 
most careful handling to avoid breakage. In fact, it was about 
as hard and strong as a thin cake of chocolate. 

After the electrolytic reduction treatment, in the 2 per cent. 
caustic solution, the whole object was dried and then covered 
with a high-grade mat lacquer, commonly used for silverware, 
such as the lacquer known under the trade name Zapon Aqua- 
nite. As the specimen now stands (Fig. 1b) there is a striking 
contrast between the rough, green, untreated upper half and the 
brick-red reduced lower half, showing the details. The mat 





Fig. 1b 


BRONZE VASE (Serial No. 25) 


26 RESTORATION OF ANCIENT 


lacquer, besides protecting, strengthens such an object and 1s 
itself practically invisible. 


BRONZE SSL KLE (Serzal No. 69) 


The results obtained in this case were almost spectacular, and 
illustrate the possibilities of treatment of a piece of bronze so 
completely mineralized that no metal core is left. The object is 
a bronze strip and when received from Rome was approxi- 
mately 4 inches by 6 inches. Figs. 2a and 2b show front and 
back as received, Figs. 2c and 2d show the finished object. 

At one time this object was part of a bronze plaque which 
served some decorative purpose, probably architectural. It was 
originally surfaced on one side with gold leaf. After ages of 
corrosion all we have left is a completely mineralized copper 
oxide strip, exceedingly weak mechanically. The strip proper 
was about /% inch thick, and before treatment was covered on 
the back with an ugly green-grey crust also about % inch thick. 
The crust on the front was much thinner, and after treatment 
revealed a flimsy gold leaf and a design underneath. At once 
this question came up: why is the crust on the face so much 
thinner than that on the back? Does this mean that the gold 
leaf protected the face? 

This object was reduced as cathode in dilute caustic soda 
solution for several days. Next dilute nitric acid was used to 
dissolve away the sponge copper covering the gold. This treat- 
ment also brought out the natural bright lustre of the gold leaf. 

The dilute acid treatment has no bad effect on the oxide core 
other than to give it a greyish tint (due very likely to the forma- 
tion of a meta-stannic acid ). The grey color of the oxide helped 
to distinguish it from the undissolved red spongy copper, and 
enabled one to decide when the acid treatment was completed. 
Immersing in dilute ammonium acetate solution and drying at 
4o° Centigrade in an oven added a desirable greenish tint to the 
oxide layer. This bronze strip or plate was finally lacquered 
with a good mat lacquer. 





40, 2a 


BRONZE STRIP—FRONT (Serial No. 69) 


(69 ON 1049S) MOVA—dINLS AZNOUA 
pe Sly 





BRONZES AND OTHER ALLOYS 20 


BRONZE FRAGMENT  (Seréal No. 84) 


Figs. 3a and 3b show this specimen before and after electrol- 
ysis in dilute caustic solution. The results are about the same 
as for the bronze plaque above. In this case a bronze core was 
found underneath the copper oxide layer. The specimen was 
finally given a patina (of a green tint) and lacquered. 


BRONZE BRACELET (Serial No. 17) 


As received (see Fig. 4a), we have an oval bracelet, bent on 
about a 34-inch radius. It was covered with a hard green crust, 
and at one point we found a large lump, like hard enamel, 
badly cracked. The photograph shows this lump at about 1% 
inch from the end. After electrolytic reduction in caustic soda 
solution and careful rubbing with a stiff bristle brush, we bring 
to light a pretty little bronze bracelet as shown in Fig. 4b. In 
this case we have a rather hard core with most of the design 
intact. A snake head is plainly seen at each end. As is usually 
the case a deep pit was found underneath the enamel-like lump. 
In general a conspicuous growth of crust indicates severe cor- 
rosion beneath. 


BRONZE LAMP (Serial No. 32) 


Figs. 5a and 5b show this lamp as received. We see front and 
back covered with a hard, lumpy, green patina. The bowl is 
2% inches in diameter. There were no suggestions of detail in 
design. After electrolytic reduction the surface was covered 
with brown spongy copper. This was carefully brushed off with 
a stiff brush. Figs. 5c and sd show the lamp after treatment. 
After brushing, the surface was of a dull copper color. 


BRONZE COINS (Serial No. 48) 


Individual bronze coins so badly encrusted as to be illegible 
have been successfully treated by reducing electrolytically in 


24 RESTORATION OF ANCIENT 


caustic solution followed by a gentle rubbing. A good example 
of such treatment is shown by Figs. 6a and 6b, which show a 
bronze coin (Lab. No. 48) before and after treatment. The 
strangest thing about coins reduced in this way is that a freshly 
reduced coin shows the design best when wet. A clean dry 
treated coin is hard to decipher. The above coin was artificially 
patinated to bring out the design. Old bronze coins sometimes 
seem not to respond to treatment. We must remember, how- 
ever, that coins were often badly worn while in use; these had a 
smooth surface before the actual corrosion in the soil started. 
Usually nothing is gained by examining such worn coins under 
the microscope. The appearance to the unaided eye is nearly 
always the best test. Photographing a treated coin while wet 
will show the design even better than visual examination. 


BRONZE FIBULA (Serial No. 67) 


This small decorated fibula is about 3 inches long, and when 
received it showed a pretty pale blue original patina under a 
thin sandy crust. In one or two spots the patina appears clean, 
bright, and enamel-like. On boiling in concentrated solutions of 
ammonium acetate for a few minutes, we removed all traces of 
the sandy crust and exposed the original design, a series of 
grooves or rings. Figs. 7a and 7b show this fibula or handle 
before and after treatment. In this case we decided not to use 
electrolysis and tried out a very weak chemical solvent (am- 
monium acetate), which gave great promise on other small 
bronze fragments. We suggest that this solution be tried along 
with the chemical solvents recommended by Dr. Scott for mild 
cases of corrosion such as this fibula. 


Gold and Silver Ornaments 


Much to our surprise and pleasure, several heavily encrusted 
ancient specimens, classed as copper-tin bronzes, turned out to 





BRONZE FRAGMENT (Serial No. 84) 





Fig. 4a Fig. ab 


BRONZE BRACELET (Serial No. 17) 








Fig. 5c 
Front 





Fig. 5d 
Back 
BRONZE LAMP (Serial No. 33) 


BRONZES AND OTHER ALLOYS 925 | 


be silver or gold (or the respective coin alloys) on treatment. 
These will be described in detail below. 

We generally assume that gold and silver are corrosion-proof. 
Such is not the case, however, if these metals are hardened or 
debased by alloying with copper. Dr. Scott in his 1921 report, 
page 7, most ably describes the mechanism of silver alloy cor- 
rosion, and gives some excellent chemical methods for treating 
it. Rathgen, pages 49 and 148, also goes into the theory of cor- 
rosion of silver alloys. 

In brief, a silver alloy containing copper, after centuries of 
burial in the earth, subject to the various agents of corrosion, 
will deteriorate. Depending upon the fineness of the silver, we 
shall get anything from a thick, hard green crust to a soft white 
or lilac film or patina. The same holds true for gold alloys, only 
not to the same degree. For pure gold or pure silver, our remarks 
do not hold, but objects of the chemically pure metals are very 
seldom found. It is our opinion that gold and silver objects, 
hardened or alloyed with copper, and subjected to underground 
corroding conditions, will undergo a copper migration or out- 
ward copper diffusion. Let us call this ““decuperification” of the 
alloy. This action seems analogous to the dezincification of brass 
condenser tubes. Perhaps aided by the membrane action of the 
first formed tarnish layer, or patina, we get a gradual outward 
migration of the copper originally alloyed with the more noble 
metal. After centuries of corroding influences, we have left, not 
a silver or a gold alloy, but an object consisting of three layers, 
(1) an inner core of porous noble metal, (2) an intermediate 
layer consisting mostly of copper, and (3) an outer crust of 
green oxy-compounds of copper. 

The inner core of “decuperized”’ gold or silver will be more or 
less porous and weak mechanically, depending upon the extent 
of the corrosion. It is this inner core, however, which carries and 
preserves the details of design. The intermediate layer, of red 
secondary copper has migrated from within, and must be re- 


28 RESTORATION OF ANCIENT 


moved. The outer green crust is usually very unlovely and is 
best removed by electrolytic reduction in caustic soda. 

The simplest way to outline such a treatment is to describe in 
detail below the restoration of several silver bracelets, silver 
coins, and gold specimens. 


SILVER BRACELET (Serial No. 1) 


After cleaning, this bracelet weighed 57 grams and was 14 inch 
(6 mm.) thick. The radius of curvature was about 114 inches 
(32 mm.). When received, it was covered with a hard green 
crust, from 4%. to % inch (1.6 to 3.2 mm.) thick, and was 
catalogued as “bronze.” The crust was unattractive and to all 
external appearances, it seemed to encase a bronze object. In 
this instance the treatment was chiefly electrolytic in 2 per cent. 
caustic soda solution. This resulted in a reduction of the green 
crust to spongy copper which was readily brushed off. However, 
we found beneath the spongy copper a sheath or envelope of red 
or secondary or reprecipitated copper that had presumably dif- 
fused from within. This 1s a good example of “‘decuperification”’ 
of a silver alloy. When this layer of secondary copper was re- 
moved partly as anode in very dilute sulfuric acid, and partly . 
mechanically, a smooth white silver surface came to view. Fig. 8 
shows the restored bracelet. Note the four grooves cut near each 
end. These were evidently made with a small chisel, as each 
individual cut is apparent. Possibly a setting formerly fitted 
between the ends. Through an oversight no photograph was 
made of the bracelet before treatment. 


SILVER BRACELET (Seng 


This was made of round silver alloy wire, 34 inch (5 mm.) 
thick, bent on a 1-inch (25 mm.) radius. Fig. ga shows its con- 
dition when received, classified as ‘‘bronze”’ ; it was covered with 
a hard green crust. After cathodic reduction in dilute caustic 
soda solution, the loose spongy copper was brushed off. Next, 





Fig. 6a Fig. 6b 


BRONZE COIN (Serial No. 48) 





Fig.7a 





Fag. 7b 


BRONZE FIBULA (Serial No. 67) 





Fig.8 


SILVER BRACELET (Serial No.1) 





Fig.oa Fig.9gb 


SILVER BRACELET (Serial No. 18) 


BRONZES AND OTHER ALLOoys 31 


the secondary copper was dissolved in weak ammonium chloride 
solution. The silver core in this case was very weak mechan- 
ically, and broke at several points owing to some slight strain in 
handling. Again we have a decuperized core of spongy silver, 
soft and mechanically weak. Fig. gb shows the restored bracelet. 


SILVER COINS § (Serials Nos.74.and77) 


Two groups of silver coins were subjected to treatment. These 
groups consisted of thirteen coins very tightly cemented together 
by the products of corrosion, a mixture of silver and copper 
compounds. It is possible that originally they had all been 
united in one lump which was partially broken during or after * 
excavation. The mass was completely covered with a hard 
green crust, and its appearance suggested nothing other than 
bronze coins, which the authorities of the Metropolitan Museum 
assumed them to be. Figs. 10a and 10b show them as received. 
They were given various treatments, mainly chemical, as out- 
lined by Dr. Scott. One group was first reduced as cathode in 
2 per cent. sodium hydrate solution. 

The results of the treatment are shown in Figs. 1oc and 1od. 
The large coin at the bottom is of bronze. The coins were found 
in all stages of decomposition. The best preserved are shown in 
the photographs. Some of the coins are covered superficially 
with a sheathing of soft, spongy silver which resisted any 
attempt to remove it by chemical or mechanical means without 
harming the coin design beneath. 

It is very difficult to give any hard and fast rule for treatment 
of silver coins in such an advanced state of corrosion. Boiling 
with formic acid, followed with boiling in acidulated ammo- 
nium and stannous chlorides as outlined by Dr. Scott, works 
well in many cases. In the 1922 report of the British Scientific 
and Industrial Research, pages 63-67, a brief report is made by 
Dr. Scott, as follows: 

“The mixture of ammonium and stannous chlorides in acid 


BD. RESTORATION OF ANCIENT 


solution has proved particularly effective in the treatment of 
coins that have been encrusted and stuck together by corrosion. 
The reagent enables the coins to be completely separated with- 
out mechanical force and, moreover, renders their inscriptions 
easily legible.”’ 

We are of the opinion that the presence of the large bronze 
coin was the cause for most of the severe corrosion. In other 
words, had these silver coins been buried out of contact with 
the bronze coin, the corrosion would not have been so marked. 
This seems borne out by the fact that the coins nearest the 
bronze coin are the most corroded. 


HANDLE OF A GOLD STRIGIL “Senay 


We have received but one gold antique for treatment. Figs. 
11a and 11b show this specimen before and after electrolytic re- 
duction in caustic solution and treatment with dilute nitric acid 
to remove spongy copper. Note the clearness with which the 
small Greek inscription has been brought out. Here, as in the 
silver ornaments, we were troubled in spots with a tight-sticking 
layer of secondary copper which is difficult to remove. We 
found that an electrolytic reduction in caustic soda, followed by 
boiling in ammonium stannous chloride solution, is most effec- 
tive for objects of this type. 


GREEK BRONZE GOAT  (Seréal No. 100) 


As received this little bronze goat was covered with a hard 
grey-green crust. The fore feet were encrusted or cemented to- 
gether. [here were scratches on parts of the body indicating that 
attempts had been made to remove the offensive crust by me- 
chanical means. The goat was submitted for three days to the 
electrolytic reduction process in 2 per cent. Na OH solution. 
The treatment worked wonders, as is shown in the accompany- 
ing photographs (Figs. 12a and 12b). 





Fig. 1ob 


SILVER COINS (Serial Nos. 74 and 77) 





Fig. iod 


Fig. toc 


SILVER COINS (Serial Nos. 74 and 77) 


BRONZES AND OTHER ALLOYS 35 
DROINZE CAT WITH KITTENS: “(SertalNo.io2) 


This is an Egyptian bronze. The surface was badly corroded 
and covered with an ugly grey-green crust. The bronze had also 
suffered mechanically, being warped and cracked. The heads of 
two of the kittens had been broken off years ago and the frac- 
tured parts were badly corroded. 

The object was given the electrolytic caustic soda treatment 
and the results obtained are indicated in the photographs (Figs. 
ieaand 13b). 


COPPER PLATE (Serial No. 26) 


An excellent example of restoration is shown by the photo- 
graphs of a copper tray or platter about 6 inches (15 cm.) in 
diameter. This tray, as received, was very badly corroded and 
covered with an ugly, hard green crust hiding all details. After 
electrolytic reduction and restoration the almost complete de- 
sign engraved in the copper was revealed. The photographs 
(Figs. 14a and 14b) give a view of the front, before and after 
treatment. 


SievhUb DLE sOR SSIS a Serzal No237) 


This is another Egyptian bronze, lent to us for experimenta- 
tion by Mr.V. Everit Macy (Figs. 15a, 1 5b before and 1 §c, 15d 
after treatment ). It is about 11 inches high and weighs approx1- 
mately 3 pounds. The white spots in the “before”? photographs 
are “‘diseased”’ regions consisting of copper carbonate plus chlo- 
ride. The left hand of Isis was very badly corroded. The crust 
over the remaining portions of the body was hard and dark green 
in color. The statuette was suspended in an earthenware tank 
containing 14 litres of 2 per cent. Na OH solution. Electrical 
connections were made to various parts of the figure, without, 
however, cutting through the crust. In all of our work great pains 
were taken never to disturb the crust or patina before treat- 
ment. The current was 114 amperes and the first treatment 


30 RESTORATION OF ANCIENT 


lasted seven days. The second electrolytic treatment lasted 
fourteen days. The statuette was then washed in warm water 
and the loosely adhering particles of reduced metal and foreign 
material were brushed off. The details were revealed in a most 
striking fashion as is shown in the photographs. Details of de- 
sign such as the necklace, the hair and headdress, toe and finger- 
nails, etc., which had been completely hidden by the crust, were 
now distinctly revealed. The diseased portions were repaired by 
the same process except the one on the left thigh, where the 
crust had disappeared before receipt of the statuette at the 
laboratory. 

The statuette after treatment was patinated by submitting it 
in an enclosed chamber to fumes of ammonia and acetic acid. 
This patinating method is discussed more fully below. 


Patinating Restored Bronze Surfaces 


Most of the bronzes that have been investigated at our lab- 
oratory were covered with greyish-green or brownish-green 
granular crusts of an unattractive, inartistic appearance. Those 
bronzes that have an enamel-like, hard green, blue, or red patina 
require, as a rule, very little if any special cleaning. The hard 
glassy patina has preserved the details in design and upon re- 
moval of the outer clayey or sandy crust, often by merely 
washing with warm water or with weak salt solutions, the beau- 
tiful patina underneath is revealed. At times gentle brushing 
with a soft bristle brush will be necessary to remove the last 
traces of clay or sand. 

Bronzes that do not have this hard protective patina are 
almost always covered with a loosely compacted unsightly 
crust. Of course it will be at once appreciated that it is impos- 
sible to bring forth from under this unsightly crust a hard, at- 
tractive, natural patina where it no longer exists. The inev- 
itably unlovely appearance of such bronzes after they have been 
through the treatment for the removal of the ugly crust is a 





Fig. 11b 
HANDLE OF A GOLD STRIGIL (Serial No. 83) 








semen 


Fig. 12b 
GREEK BRONZE GOAT (Serial No. 100) 








Fig. 13a 





Fig.13b 


BRONZE CAT WITH KITTENS (Serial No. 102) 


BRONZES AND OTHER ALLOYS 39 


matter of common experience no matter what the method for 
the removal of the crust has been. Dealers as well as museum 
authorities, recognizing the fact, have sought to overcome it by 
supplying an artificial patina, often deceptively successful. 
There is no doubt that a large percentage of the ancient bronzes 
in public and private collections have been artificially pati- 
nated. In view of the circumstances, the procedure seems to be 
amply justified, provided that the bronze itself suffers no injury 
from it and that the patina can be readily removed if desired. 
Upon that understanding, Mr. Edward Robinson, the Director 
of the Metropolitan Museum, wished us to carry on experiments 
to the point of determining the most desirable form of artificial 
patination to be applied to bronzes requiring it. 

Upon treating many of the bronzes by our electrolytic reduc- 
tion methods a smooth metallic or metallic-like surface is ex- 
posed. This surface can be much improved in appearance by 
developing over the surface a patina very similar to that met 
with naturally. We felt that if we reproduced the natural pati- 
nating conditions in the laboratory and intensified these we 
ought to obtain effects very similar to those obtained after years 
of exposure. Accordingly, we subjected bronzes to the attack of 
various gases such as ammonia, carbon dioxide, sulfuretted hy- 
drogen and acetic acid. This investigation is still in progress, but 
the best results up to date have been obtained by exposing the 
bronze object to a simultaneous attack of ammonia and acetic 
acid gas. Beautiful bluish-green patinas are obtained which lose 
the bluish tint upon heating the bronze in an oven at about 110° 
Centigrade, bringing about a gradual change. 

We do not favor the use of any chloride as we consider chlo- 
rine one of the chief destructive agents of bronzes and the cause 
of much of the so-called bronze disease (see below). 

Patinating by the gas method is far more satisfactory than 
the older methods of dipping the object in ammonium chloride 
solution or painting the object with copper carbonate, or—as 


40 RESTORATION OF ANCIENT 


has been done in some cases—painting the object with a green 
or blue aniline dye. The gas methods bring out artistic effects 
which are practicalky impossible by any dipping or brush 
methods. 

The procedure we recommend is to place the object in an air- 
tight cell or box at the bottom of which are two small vessels or 
jars, one filled with concentrated ammonia solution and the 
other with strong acetic acid. It 1s not necessary to heat the 
chamber. The time required for a satisfactory patina is from ten 
to thirty hours, although even longer treatment will be found 
necessary in individual cases. To render this artificial patina 
proof against weather conditions such as exist in New York, the 
surface must be protected with a mat laquer or with a solution 
of beeswax. It is important to note that a long exposure of the 
bronze surface to a weak corroding atmosphere will bring about 
more artistic and lasting effects than a short exposure to strong 
reagents. We therefore do not approve of heating the reagents 
or the cabinet itself during the gas treatment. 

Horns in his book on Metal Coloring and Bronzing recom- 
mends “‘a judicious and thorough scratch brushing” in order to 
secure uniformity of surface. We have tried this but found that 
the results were far from satisfactory. The beautiful artistic 
green, blue, brown effects brought about by the gas treatment 
are more or less destroyed. 

In some cases we have found that a preliminary gas treatment 
in sulfuretted hydrogen and a subsequent treatment in ammonia 
and acetic acid gas produced a dark green to black patina. On 
the other hand, we have found the action of carbonic acid gas 
alone too slow for practical purposes ; a mixture of carbonic acid 
gas and acetic acid gas, with the former in large excess, is more 
active. Chlorine gas or volatile chlorine compounds such as 
‘muriatic acid and salammoniac give very beautiful effects, but 
we do not recommend any of these on account of the treacher- 
ous nature of chlorides in the patina. 





Fig. 14b 


COPPER PLATE (Serial No. 26) 


42 RESTORATION OF ANCIENT 


The Bronze Disease 


We have referred to this bronze disease repeatedly above. 
Very often it is due to a phenomenon similar to the well-known 
“tin pest,’ when tin is gradually transformed from one allo- 
tropic modification to another. We have found that the usual 
immediate cause of the bronze disease is the presence of a trace 
of chloride, and the action is primarily electrolytic. 

We have closely studied a number of bronzes that were in- 
fected with the bronze disease. The first intimation that the 
object is infected is usually the appearance of a few fine grains 
of light green dust on the base or platform upon which the 
bronze is mounted. These grains are so fine and so light that 
unless the bronzes are encased in glass they will be easily blown 
away. Upon carefully examining the bronze object and tracing 
the origin of the grains of light green dust we find an area, often 
not larger than that of a pin-head, which is rough in appearance 
and free of all lustre as compared with the rest of the surface of 
the bronze. Taking a fine needle we note that the infected area 
is comparatively soft. If neglected the area will grow at a more 
or less rapid rate, depending upon conditions such as the pres- 
ence of moisture in the surrounding air. 

How the bronze disease actually starts at one or the other 
spot on the surface of the bronze it is very difficult to determine. 
We have observed that when a mere speck of lampblack or coal 
soot is deposited on a sheet of bright clean copper and the sheet 
exposed to our laboratory atmosphere that the black speck soon 
becomes the centre of a corroding area. Whether it is a trace of 
sulfurous or sulfuric acid originally present in the carbon speck 
that accelerates the corrosion of the copper in the immediate 
vicinity of the speck, or whether it is a trace of one or the other 
constituents of the atmosphere such as ammonia, sodium or 
ammonium chloride, or a sulfide which has been adsorbed and 
condensed by the carbon speck and brought about an acceler- 





Fig.15a Fig. 15¢ 


STATUETTE OF ISIS—FRONT (Serial No. 87) 





Fig. 15b Fig. 15d 


STATUETTE OF ISlo— BACK (Serial No. 87) 


BRONZES AND OTHER ALLoys 45 


ated corrosion, is very difficult to determine, on account of the 
very minute quantities of material dealt with. 

Copper sheets given a thin film of ammonium chloride and 
kept in a fairly moist atmosphere soon become pitted, due to a 
slow but continuous action of the chloride. Only a very minute 
amount of chloride is needed to convert a large mass of copper 
into malachite or azurite in the presence of moisture and car- 
bonic acid as found in the air. 

For combating the bronze disease various methods have been 
suggested in the past. A common method is to coat the affected 
part with pitch or bitumen but this is very unsatisfactory as it 
leaves an ugly black spot. We recommend the following pro- 
cedure: clean the affected parts thoroughly by the electrolytic 
method described above, using the 2 per cent. sodium hydrate 
solution. Then wash and dry carefully and apply a mat lacquer 
(cellulose acetate). This lacquer should be renewed at least 
once in four years. : 
Conclusion 

Ancient bronzes that had been badly corroded have hitherto 
been subjected to various treatments for the purpose of restor- 
ing their surfaces. A new, comparatively simple method has 
been developed in the investigations described above, which has 
been successfully applied to a wide variety of bronze objects. 
The method is primarily an electrolytic one and its chief advan- 
tage lies in its being absolutely safe as compared to older acid 
dip methods or the chisel and hammer method. 

The new method we have described in detail and the accom- 
panying photographs show the results that have actually been 
obtained. 

Methods for patinating and for combating the bronze disease 
are only briefly described because this part of the investigation 
is still in progress, and what we should regard as absolutely satis- 
factory methods have not yet been achieved. We hope, however, 
to make these the subject of a subsequent report. 


46 RESTORATION OF ANCIENT 


APPENDIX 


EXAMINATION OF BRONZES TO ESTABLISH, IF POSSIBLE, 
THEIR GENUINENESS 


In the course of our investigations on ancient bronzes, we 
have had occasion to examine a number of objects concerning 
which there was some uncertainty as to whether or not they 
were genuine antiques. It need hardly be mentioned that there 
are a large number of spurious antiques sold to people who are 
not familiar with even the simplest means of determining the 
composition and age of the object and deciding whether or not 
the claims made by the dealer are fact or fiction. 

Just as there are means and methods of detecting a counter- 
feit “Rembrandt” so likewise are there methods of detecting an 
imitation or spurious “antique” bronze. Although it is true that 
absolute identification of the fraud is not always possible, in the 
large majority of cases we can by a systematic method of phy- 
sical, chemical, and metallographic analyses determine whether 
or not the bronze or other alloy is of comparatively recent origin. 

We have divided our method of investigation and procedure 
into three distinct steps: 


1. Microscopical examination of the patina or crust. 
2. Metallographic examination of the metal core. 
3. Chemical analysis of the metal. 


In the examination of the patina we may find distinct crystal- 
line growths of malachite or azurite, the tiny crystals partially 
imbedded in the underlying oxide film. Such crystalline growths 
are most difficult to imitate and patinas of recent origin are 
almost always amorphous or non-crystalline. Nevertheless, the 


BRONZES AND OTHER ALLOys A7 


patina may be recently formed or applied and yet the bronze 
may be genuine—in fact, very many of the genuine bronzes 
exhibited nowadays have patinas of recent or artificial for- 
mation. 

Accordingly, if the microstructure of the malachite or azurite 
particles in the crust or in the patina 1s distinctly crystalline and 
interlaced with the copper oxide layer, the bronze is very likely 
genuine. But an amorphous malachite deposit does not neces- 
sarily imply that the underlying bronze is not genuine. 

Under the outermost coating of malachite or azurite or clayey 
material there usually is found, in the case of genuine bronzes, a 
layer of copper oxide, reddish-brown to reddish-black in color. 
This layer has been repeatedly referred to in the body of this 
report. It is usually of considerable thickness and is not readily 
applied by any rapid artificial process. 

In the metallographic examination of the bronze we find ad- 
ditional proof as to whether or not the bronze is genuine. Taking 
a very fine slice of metal from some inconspicuous part of the 
bronze, and preparing this by polishing and etching for micro- 
scopical examination, we note in the case of a genuine bronze 
that there is a gradual change in structure as we pass to the 
outer exposed edge of the bronze; furthermore we will some- 
times note changes in the size of certain crystal components of 
the bronze, changes that are brought about by a very slow proc- 
ess of “‘annealing.”’ We have referred to the process of decuperi- 
fication in the body of this report and will cite another definite 
case below. 

Chemical analysis must be carried out with great care. We 
can support the findings of our metallographic investigation by 
determining chemically that the main body or interior of the 
bronze has a decidedly different composition from the layers 
under the outer oxide or patina surface. 

How the above method of investigation and detection may be 
applied can best be illustrated by the two examples cited below. 


48 RESTORATION OF ANCIENT 


SILVER BRACELET (No. 22.50.3) 
(a) The Pendant Pan 


This is a small silver pendant attached to a silver link brace- 
let (Figs. 16a and 16b). Upon examination under the micro- 
scope we found on the back and on the arms of the image of Pan 
irregular deposits of tarnished metal. Further study indicated 
that these deposits were of secondary formation, very similar 
to those reported in the case of the two silver bracelets (see 
page 23). These irregular deposits on the back and on the arms 
are due to a process of corrosion which we have termed “‘decu- 
perification.”’ The process of decuperification is very similar to 
the process of dezincification, commonly met with in the case of 
brass, where the surface of the alloy decomposes and as a result 
of the operation of corrosion, pure copper is found on the surface 
of the brass, the zinc having disappeared into the crust or patina 
or beyond. In the case of the image of Pan and the links to which 
it is attached, the alloy consists of silver with small percentages 
of copper. During the process of corrosion, there has been a ten- 
dency for the copper to pass out of the surface and leave pure 
silver behind. As the result of this process, silver is redeposited 
in irregular patches and these pure silver patches are tarnished 
by the sulfur gases of the atmosphere. Besides copper, there may 
be other metals in the alloy of which the image is composed, 
which tend to make the metal more resistant to corrosion. At all 
events the secondary deposits of silver on the back and on the 
arms of Pan are darker, in some places jet black, as compared 
with the rest of the body of this little image, the pure silver tar- 
nishing more readily than the original alloy. We surmise that 
the image had been cleaned before we received it and that it was at 
one time covered with a dark green crust. Patches of the green 
patina are still present; for example, there is a small patch of 
green patina underneath the beard, also examination shows 
green patina on the right arm. These green patches indicate the 





Fig. 16b 
The Pendant Pan 


SILVER BRACELE (GVo.22.560:43) 





MIRROR (No. 22.50.1) 


BRONZES AND OTHER ALLOYS Sal 


presence of copper in the alloy. We hesitated to make a chemical 
analysis of the image since the bronze was too small and we were 
afraid we would deface it. 


(b) The Chain 


The links and the hook are rather odd and not usually met 
with in modern times. In the case of a number of the links, it is 
difficult to see where the metal has been forged together. The 
workmanship has been very careful and accurate. Upon closer 
examination of the links and of the ring connecting the two 
halves of the chain to which the pendant is attached, we ob- 
served the same type of secondary deposit of silver referred to 
above which has turned black on exposure to sulfur gases. The 
second link to the left of the ring shows this black silver most 
strikingly. These secondary deposits of silver take many years to 
form and it would require a very clever imitator to bring about 
results which would at all resemble the metal deposits on the 
chain and on Pan. Our conclusions, therefore, were that the 
chain and pendant are composed of a silver-copper alloy. This 
alloy has been decuperized on the surface and deposits of pure 
silver have formed. Since this action is extremely slow, since 
the patches are very natural in appearance, and since, further- 
more, the chain and pendant are very similar in characteristics 
to the two silver bracelets previously examined, we therefore 
concluded that the chain and pendant were very old. 


MIRROR (No. 22.50.1) 


The mirror (Fig. 17) had been originally turned over to a 
“repairer,” by whom it was cleaned. Small tufts of the cotton 
used in cleaning still adhered to the surface of the mirror, in 
particular on the reverse side. The mirror was at one time 
fastened to a wooden backing or support and plaster of paris 
had recently been applied to hold the metal to the wood. 

Samples of the patina, the wood, and the metal were taken. 


52 RESTORATION OF ANCIENT 


In sampling the patina it was very difficult to find portions that 
had not been disturbed or altered during the cleaning process or 
the recent application of the plaster of paris. ‘wo small slivers 
of metal from the back of the mirror were taken. These were 
almost black in appearance. 

The patina was carefully examined under the microscope. We 
found malachite in abundance, some of which was distinctly 
crystalline in structure, showing the fine needlelike prisms. 
This crystalline structure was a very strong indication that the 
patina was not of recent origin. It is practically impossible to 
produce patina of crystalline malachite artificially. The crystal- 
lization process is very slow and the malachite or copper car- 
bonate produced by ordinary chemical manufacturing processes 
is amorphous in structure. Ihe element of time is a very im- 
portant factor in producing malachite crystals. 

Besides malachite we observed very fine crystals of calcite 
and, although these too were crystalline, the proof as to age is 
not so strong as it is in the case of the crystalline malachite. 

The front face of the mirror frame was protected by a thin 
layer of gold and only where the gold had separated from the 
underlying bronze was excessive corrosion apparent. The two 
bronze slivers were covered with a very dark oxide of copper 
and over this in spots were small growths of malachite. The 
metallographic analysis of a sample of the bronze indicated that 
the alloy was very old, although the evidence is not so strong as 
in the case of the patina. 

Owing to the contamination of the patina samples with cot- 
ton fibre used by the dealers in cleaning the mirror, the chem- 
ical analysis was not reliable and no definite conclusions could 
be drawn. 

The wood sample was examined under the microscope. The 
fibrous structure was highly characteristic of very old wood. The 
cells were distinctly visible and small white crystals of mineral 
matter were scattered throughout. There seems very little doubt, 


BRONZES AND OTHER ALLOYS 53 


to our minds, that the wood is of ancient origin. Of course, 
this does not necessarily prove that the mirror is of ancient 
origin. However, on the basis of our microscopical examination 
of the metal and of the patina we concluded that the mirror was 
of ancient origin. 


OF THIS BOOK 
ONE THOUSAND COPIES 
WERE PRINTED 
MAY, 1925 
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